The disturbance of circulatory dynamics and breakdown of the blood retinal barrier will be evaluated in monkeys made diabetic by the streptozotocin technique. Findings of fundus photography, fluorescein angiography and vitreous fluorophotometry will be correlated with histologic and ultrastructural changes in the retina. The nature of the barrier breakdown will be studied by horseradish peroxidase labeling technique and by the rate of escape of fluorescein labeled dextrans of various molecular weights. Therapeutic effect of the reversal of diabetes by islet cell transplantation will be evaluated.